Hydraulic brake systems are subjected to pressure created when brakes are applied. More particularly, applying pressure to the brake pedal of a braking system creates pressure in the master cylinder piston bore. In a traditional hydraulic braking system, the pressure created in the master cylinder piston bore is typically generated by a metal or plastic piston sealed by rubber cup seals, such as primary and secondary cup seals.
In higher pressure braking systems, such as Anti-lock Braking Systems (ABS), the pressure in the piston bore is significantly higher during braking operation than in a non-ABS application. In addition, when ABS is activated, the piston in the piston bore oscillates rapidly back and forth exerting substantial stress on the cup seals. The higher pressure, along with the violent reciprocation of the piston during ABS activation, can result in premature wear as well as damage to the cup seals, which can result in a catastrophic failure.
One method of accommodating the oscillating high-pressure effects of ABS is to utilize a central valve positioned inside the piston bore to adjust the internal pressure. The use of a central valve is well known in the art. Although the central valve can reduce the wear and damage of a cup seal by allowing bore cavity pressure to vent through the piston, numerous drawbacks exist with a central valve system. More particularly, a master cylinder assembly that utilizes a central valve contains several additional parts, which increase cost of manufacturing. In addition, the required valve actuation of the central valve can decrease the reliability of a master cylinder assembly, as additional moving parts are required to operate. Also, by adding the additional moving parts, various manufacturing tolerances are introduced that can create a large variation of dead-stroke among master cylinder assemblies manufactured under identical manufacturing specifications. Further, the use of a central valve can increase lead-stroke distance that a piston and a cup seal travel before pressure can begin to build, which reduces the responsiveness and firmness sought during actuation of the master cylinder assembly, leaving a less responsive and undesirable “spongy” feel when the braking system is actuated.